New DelhiA sports bike left behind at the scene of crime has helped Delhi Police crack the murder of a father-son duo at their eatery in Najafgarh on Wednesday night.Four men, including one with a criminal background, have been arrested for the killing that took place over a petty argument over which direction an air cooler at the eatery should be turned.Interrogation of the accused has revealed that one of the men had returned to the crime spot within minutes of the double murders to retrieve the motorcycle left behind.The victims, 52-year-old Shyam Verma and his son Mayank, ran Sangeeta Dhaba located in Nangli Sakrawti in Najafgarh. On Wednesday evening, their customers included four men who occupied a table near the roadside.As the men were ordering dinner, one of them wanted the lone air cooler at the eatery to be turned towards him. An argument ensued after Mayank said that there were other customers as well and objected to the cooler being turned.“The group of men wanted exclusive treatment at the dhaba. When they were denied the air cooler, they were angry. One of them whipped out a pistol and shot Mayank,” said MN Tiwari, DCP (outer).When Mayank’s father rushed to the aid of his collapsing son, he too was shot in his neck. While Shyam died at the spot, Mayank succumbed a few hours later.The firing caused panic at the dhaba with many customers running out to save their lives. A number of customers left their motorcycles behind as they fled to safety.One of the motorcycles left behind was an orange KTM Duke, which belonged to Karim Khan, one of the alleged killers. “In a hurry to get away, he had misplaced the bike keys. So, he could not flee with the bike after the murders,” said the DCP.Desperate to wipe out any clues that could lead the investigators to them, the killers decided to retrieve the motorcycle immediately after.“Around 15 minutes after the murder, Khan returned to the dhaba to search for his bike keys but couldn’t find it,” said the DCP.Khan continued to look for the keys even as police arrived at the dhaba. Soon, other customers returned to take away their bikes one by one after police verification, but Khan was left helpless. He finally left the spot more than 30 minutes later.Later, one of the customers who had witnessed the murders told investigators that the KTM Duke motorcycle belonged to one of the killers. Police immediately seized the motorcycle and began tracing its owner.The cops found that the bike had been sold at least four times. The last sale of the motorcycle was done illegally, so it took the police a while to trace the last owner.Khan was finally nabbed on Thursday. He turned out to be a bike mechanic. At his instance, the other three killers — Raja, Amit and Monu —were nabbed on Saturday. While Raja and Amit are bike mechanics, Monu runs a dairy business. Monu was found to be involved in five previous crimes, which includes attempt to murder and molestation. The DCP said it was Monu who fired the gun.